Mitochondrial changes related to yolk formation in oocytes of a viviparous teleost

Abstract

Mitochondria have been implicated in yolk formation in amphibians the oocytes of which were shown to contain intramitochondrial yolk crystals. We recently observed, however, in the viviparous teleost Xiphophorus helleri, that mitochondria seem to intervene in yolk accumulation in a different way by developing invaginations of their contour which enclose materials that are later incorporated into yolk globules. Ovaries were fixed in glutaraldehyde—osmium or osmium alone and epon embedded.In 15 day-old females, at leptotene, oocyte mitochondria appeared closely associated with granulo-fibrilar perinuclear masses identical to other masses situated on both sites of nuclear pores or near the nucleoli (Fig. 1). Granules in all these masses measured 12 nm in diameter and were similar to those forming the nucleolar pars granulosa. At dictyate, while nucleoli became vacuolated and ring-shaped, mitochondria surrounded the granulo-fibrilar masses (Fig. 2), though by the second half of this stage they appeared detached from those masses and showed constrictions and sinuous contours.